Electron-microscopic studies of the thymus of white laboratory rats under unfavorable environmental conditions established ultrastructural features of stromal cells of different zones of the thymus. In the cortical zones, along with classical epithelioreticulocytes, dendritic and monocyte-like cells are predominant, interdigitating cells are localized mainly in the medullary zones. Macrophages and monocyte-like cells are frequent in the corticomedullary zones.